1. Field of the Invention
The present invention relates to a semiconductor display device of active matrix type. More particularly, the present invention relates to a liquid crystal display device of active matrix type which employs liquid crystal as a display medium. Note that in the semiconductor display device according to the present invention, any other display medium (an electroluminescence device, for example) may be used as long as the optical characteristic of the medium is modulated in correspondence with applied voltage.
2. Description of the Related Art
Recently a technique has been rapidly developed which relates to the manufacture of a semiconductor device, such as a thin film transistor (TFT), having a semiconductor-thin film formed on an inexpensive glass substrate. Behind this lies an increasing demand for a liquid crystal display device (liquid crystal panel) of active matrix type.
A liquid crystal panel of active matrix type has a pixel TFT arranged for each of pixel regions consisting of several hundred thousands to several millions of pixels and are arranged in a matrix, (this circuit is called an active matrix circuit), and the TFT controls by means of its switching function an electrical charge that runs in and out of each pixel electrode.
The active matrix circuit is made up of thin film transistors utilizing amorphous silicon formed on a glass substrate.
Also there has been realized recently a liquid crystal display device of active matrix type in which a quartz substrate is utilized and a thin film transistor is formed of polycrystal silicon film. In this case, a peripheral driver circuit for driving the pixel TFT may also be formed on the same substrate on which an active matrix circuit is formed.
In addition, a technique is known in which a thin film transistor using a crystalline silicon film is formed on a glass substrate by utilization of laser annealing technique or the like. Utilization of this technique makes it possible to integrate on a glass substrate an active matrix circuit with a peripheral circuit.
A large number of notebook type computers in recent years employ liquid crystal display devices of active matrix type. In a personal computer, a liquid crystal display device of multi-gradation is required in cases such as a plurality of software are activated at the same time, and an image from a digital camera is inputted to be processed.
Further, a demand for a liquid crystal display device of active matrix type with small dimensions and high definition/high resolution/high image quality follows on recent growing popularity of portable information terminals, mobile computers, vehicle navigation systems and the like.
When providing a liquid crystal display device of active matrix type with small dimensions and high definition/high resolution/high image quality, it is a matter of significance that the pitch between pixels is made small and how minute the display gradation is made. In order to decrease the pitch between pixels, a pixel TFT has to be reduced in size while meeting difficulties. To carry out fine gradational display, on the other hand, it needs to use a driver circuit of excellent performance, which has not been realized by any circuit in prior art with the sole exception of an external IC circuit.